Problem: The scale on a map is 4cm : 3km. If the distance between two cities is 21km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 4cm on the map corresponds to an actual distance of 3km. An actual distance of 21km is the same as 7 $\cdot$ 3km. The distance between the two cities on the map is 7 $\cdot$ 4cm, or 28cm.